after some more issues with the update stuff i finally resolved it:
in the registry you can set the default installation path for third party programs (games and such) in HKLM/Software/Microsoft/Windows/CurrentVersion -> the value is something like "C:\Program Files" or so. I changed mine to D:\... which wasn't recognised by the programs i wanted to install but apparently by the windows update service. because when i changed it back to C... it worked all fine again. Stupid windows.